When machining is performed using a machine such as an NC machine tool or an NC laser beam machine, control is made such that a position of a tool (an edge tool such as an end mill) relative to a workpiece is displaced along a commanded path. This control is referred to as “trajectory control” and is generally performed by executing servo control so that an actual position of each movable axis of the machine follows a position command of each movable axis obtained from the commanded path.
As a problem in executing the trajectory control, there has conventionally been a point where an actual trajectory deviates from the commanded path due to a response delay of a control system of each movable axis. Usually, because control is executed for each movable axis of the machine, each movable axis makes movement with a servo system response thereof being delayed with respect to a position command due to an error resulting from the response delay of the control system of each axis. When a moving direction of the commanded path does not change as in a straight line, even if each axis makes movement with a delay, a trajectory of the servo-system response does not deviate from the commanded path. That is, although an error appears in a tangential direction of the commanded path, an error does not appear in a normal direction of the commanded path. On the other hand, when the moving direction of the commanded path changes as in a curved line or a corner shape, an error appears in the normal direction of the commanded path due to a delay of the servo control system of each axis.
In the following explanations, among errors with respect to the position command of a position of the servo system response, a component in the tangential direction of the commanded path is referred to as “following error”, and a component in the normal direction of the commanded path is referred to as “trajectory error”. Generally, if there is a trajectory error, a machined shape does not match with an intended shape, and so this situation is not preferable. Meanwhile, the following error does not directly affect the machined shape, and thus the following error may be frequently allowable, but if the following error is excessive, then the machining time increases, and so this situation is not preferable.
As means for limiting these trajectory errors and following errors, for example, Patent Literature 1 discloses a method in which in control of a hand position of a robot, a hand position after a predetermined sampling time is estimated, and a position command is corrected by a value of a normal vector drawn from the estimated hand position to a target trajectory, so that the hand position is maintained on the target trajectory and a trajectory error is limited while allowing a time delay. That is, among errors occurring at the time of executing control of a plurality of movable axes, the trajectory error, which is an error in a direction perpendicular to the target trajectory, namely a commanded path, is estimated, and the commanded path is corrected in an opposite direction by the amount of the estimated trajectory error, so as to correct the trajectory error, thereby limiting the trajectory error possibly occurring in a moving path.